1. Field of the Invention
The present invention relates generally to a process of producing a semiconductor layer structure which is used in a semiconductor light emitting device. More particularly, this invention relates to a process of producing a novel semiconductor layer structure which can ensure integration of a semiconductor laser and active devices, such as a photodiode, and integration of those active devices and an optical waveguide.
2. Prior Art
FIG. 1 shows one example A0 of a layer structure which is used in a semiconductor laser device. The layer structure A0 in the semiconductor laser device is realized as follows. A lower cladding layer 2 of Se-doped n-type InP (carrier density of 1xc3x971018cmxe2x88x923) having a thickness of 500 nm is deposited on a substrate 1 of n-type InP. Sequentially stacked on the lower cladding layer 2 are a 40-nm thick lower light-confinement layer 3a of GaInAsP (xcexg=1.1 xcexcm), a strained multi-quantum well structure 4 which is comprised of a 10-nm thick well layer of GaInAsP (strain: +1%) and a 10-nm thick barrier layer of GaInAsP (xcexg=1.1 xcexcm), and a 40-nm thick upper light-confinement layer 3b of GaInAsP (xcexg=1.1 xcexcm). On the upper light-confinement layer 3b is formed a 2000-nm thick upper cladding layer 5 of Zn-doped p-type InP (carrier density of 1xc3x971018cmxe2x88x923).
A 300-nm thick contact layer (not shown) of Zn-doped p-type GaInAs layer is formed on the upper cladding layer 5 and is then subjected to predetermined photolithography and etching, thereby forming, for example, a ridge optical waveguide. An unillustrated upper electrode is formed on the ridge optical waveguide and an unillustrated lower electrode is formed on the back surface of the substrate 1. This completes a semiconductor laser device.
For this semiconductor laser device with the layer structure A0 constructed in the above-described manner, the wavelength of light emitted from the quantum well structure 4 becomes 1300 nm.
To provide a semiconductor light emitting device with enhanced functions and to integrate the device, an attempt is made to integrate semiconductor layer structures having various luminescence wavelength characteristics on the same semiconductor substrate. In this approach, a slab layer structure, such as the layer structure A0 as shown in FIG. 1, is temporarily formed, part of the layer structure is etched out to the lower cladding layer 2 to locally leave the layer structure A0 whose luminescence wavelength becomes 1300 nm, then another semiconductor material is grown again on the etched-out portion to form a layer structure which shows another luminescence wavelength.
As an alternative approach, a crystallization-preventing mask of, for example silicon nitride, is formed into a predetermined pattern in the surface of layer structure having specific luminescence characteristics and another semiconductor material is selectively grown on the unmasked surface.
According to the conventional semiconductor laser devices, as apparent from the above, even if layer structures having different luminescence characteristics are integrated monolithically on the same substrate, the integrated layer structure is not formed of a single quantum well structure, but is formed of a complex of a plurality of quantum well structures which are arranged side by side, and show different luminescence characteristics, respectively.
That is, the conventional semiconductor laser devices are not designed to provide plural types of luminescence characteristics from a single quantum well structure.
The present inventors stacked an Se-doped n-type InP layer on the entire surface of the upper cladding layer 5 of p-type InP having a thickness of 100 nm in the layer structure A0 shown in FIG. 1 and measured the photoluminescence (PL) of a layer structure L in the layer structure.
As a result, the luminescence wavelength of the quantum well structure 4 was shifted toward the short-wavelength side. Specifically, with the Se-doped n-type InP layer having carrier density of 1xc3x97108cmxe2x88x923 and a thickness of 1000 nm, the luminescence wavelength of the quantum well structure 4 became 1250 nm.
The present inventors etched out the temporary Se-doped n-type InP layer to return the layer structure to the one shown in FIG. 1 and then likewise measured the photoluminescence (PL) of the resultant layer structure. The luminescence wavelength of the quantum well structure 4 did not return to the original wavelength of 1300 nm but stayed at 1250 nm.
It was also found that the luminescence wavelength of the quantum well structure 4 would be shifted toward the short-wavelength side even if the thickness of the Zn-doped p-type InP layer located directly above the quantum well structure 4 was changed or further the carrier density was changed.
After the formation of the aforementioned Se-doped n-type InP layer, heating at the layer-forming temperature was maintained for about 30 minutes. This increased the amount of the shift of the luminescence wavelength of the quantum well structure 4 toward the short-wavelength side is seen.
The following is the summary of the above experimental results.
(1) When an n-type InP layer is formed on the p-type InP layer 5 in the layer structure A0 shown in FIG. 1, the luminescence wavelength of that part of the quantum well structure which is located directly below the n-type InP layer is shifted toward the short-wavelength side for some unknown reason.
(2) The above phenomenon occurs even after the n-type InP layer is removed. That is, once the n-type InP layer is formed on the p-type InP layer, that part of the quantum well structure which is located directly below the n-type InP layer is transferred to a short-wavelength light emitting area, regardless of whether or not the n-type InP layer thereafter exists.
(3) The amount of the shift of the luminescence wavelength toward the short-wavelength side is changed by the thickness of the p-type InP layer formed on the quantum well structure or the carrier density.
(4) Keeping heating at the layer-forming temperature after the formation of an n-type InP layer on a p-type InP layer increases the amount of the shift of the luminescence wavelength of the quantum well structure that is located directly below the n-type InP layer toward the short-wavelength side.
The above new findings led the present inventors to have the following idea. If that portion of the layer structure A0 shown in FIG. 1 where the n-type InP layer is stacked on the p-type InP layer is changed, that part of the quantum well structure that is located directly below the stacked portion serves as a short-wavelength light emitting area, and the other portion than the stacked portion stays as an area which emits light with the designed wavelength of the quantum well structure. It is therefore eventually possible to form areas with a plurality of luminescence wavelengths in the same quantum well structure in a planar fashion.
Based on this idea as well as the above findings (1) to (4), the present inventors made various studies to develop a process of producing a semiconductor layer structure embodying this invention.
Accordingly, it is an object of the present invention to provide a novel process of producing a semiconductor layer structure for a semiconductor light emitting device, which can allow even a single quantum well structure to provide a plurality of different luminescence characteristics, unlike the conventional semiconductor layer structures which achieve different luminescence characteristics with a plurality of quantum well structures.
To achieve the above object, according to one aspect of this invention, there is provided a process of producing a semiconductor layer structure comprising the steps of forming a layer structure having a quantum well structure on a semiconductor substrate by epitaxial growth and stacking at least a first semiconductor layer of a first conductivity type in the vicinity of the layer structure; and stacking a second semiconductor layer having an opposite conductivity type to that of the first semiconductor layer on an entire surface or a partial surface of the first semiconductor layer, so that at least one part of the quantum well structure becomes an area which emits light with a shorter wavelength than those of lights from the other portions.
The second semiconductor layer may be removed after the layer thereof. After the stacking the second semiconductor layer, heating at the forming temperature of the second semiconductor layer may be maintained.
Specifically, the present invention provides a process of producing a semiconductor layer structure where the layer structure comprises a lower cladding layer, a quantum well structure and an upper cladding layer, the semiconductor substrate is formed of n-type InP, the first semiconductor layer is formed of Zn-doped p-type InP and the second semiconductor layer is formed of n-type InP.